1. Field of the Invention
This invention relates to thermal ink jet printers and, more particularly, to a method and apparatus for selecting pulse signals to be repeatedly applied to heating elements so as to maintain the print quality.
2. Description of Related Art
A thermal ink jet printhead selectively ejects droplets of ink from a plurality of drop ejectors to create a desired image on an image receiving member, such as paper. The printhead typically comprises an array of drop ejectors that convey ink to the image receiving member. In a carriage-type ink let printhead, the printhead moves back and forth relative to the image receiving member to print the image in swaths. Alternatively, the array may extend across the entire width of the image receiving member to form a full width printhead. Full width printheads remain stationary as the image receiving member moves in a direction substantially perpendicular to the array of drop ejectors.
The ink jet printhead typically comprises a plurality of ink passageways, such as capillary channels. Each channel has a nozzle end and is connected to an ink supply manifold. Ink from the manifold is retained within each channel until, in response to an appropriate signal applied to a resistive heating element in each channel, the ink in a potion of the channel adjacent to the heating element is rapidly heated and vaporized. Rapid vaporization of some of the ink in the channel creates a bubble that causes a quantity of ink (i.e., an ink droplet) to be ejected through the nozzle to the image receiving member. U.S. Pat. No. 4,774,530 to Hawkins, the disclosure of which is incorporated herein by reference, shows a general configuration of a typical ink jet printhead.
The reliable generation of bubbles by a heating element depends on the power per unit area generated by a heating element. When the voltage across the heating element is held constant, the power generated by a heating element decreases as the resistance of the heating element increases. The power generated by a heating element also decreases as the resistance of the driving transistor increases. Additionally, the power generated by a heating element decreases as the ambient temperature increases.
If the generated power is too low, then the heating element temperature may be insufficient to form bubbles in the nozzle. Therefore, the drops may or may not be generated by the nozzle. This could result in sporadic printing.
On the other hand, when the generated power is too high, although the heating element may form bubbles easily, the temperature of the heating element is elevated and the lifetime of the heating element is reduced. Accordingly, a high generated power reduces the longevity of the heating element.
Therefore, for at least the reasons set forth above, the power dissipated by a heating element is critical to uniform operation over time.